A variety of compounds have been investigated with respect to their usefulness as electron-donating color formers that would form color upon contact with electron-accepting compounds, and among the compounds which have been extensively studied are triphenylmethane phthalide compounds, fluoran compounds, phenothiazine compounds, indolylphthalide compounds, leucoauramine compounds, rhodamine lactam compounds, triphenylmethane compounds and triazene compounds. Such compounds may be employed in various recording systems such as pressure-sensitive, heat-sensitive, and current impressed heat-sensitive recording systems. The performance of recording materials depends largely upon the characteristics of the color formers, such as their water solubility, oleophilicity and hydrophobicity. Attempts have been made to introduce oleophilic groups into the color formers or convert them into bis forms with a view to increasing their oleophilicity to such an extent that they can be dissolved in oils in higher concentrations or in order to minimize the occurence of fogging as described, for example, in Japanese Patent Application No. 64036/83 (corresponding to U.S. Patent Application Ser. No. 599,361 filed on Apr. 12, 1984). However, the color formers into which oleophilic groups have been introduced have increased molecular weights and they produce a reduced color density for the unit weight. Color formers in the bis form are also disadvantageous in that they are difficult to manufacture and in that considerable difficulties are encountered in obtaining and purifying the starting materials.
The present inventors have heretofore made concerted efforts to solve these problems of the prior art techniques and, as a result, have come to realize that such problems can be solved by converting color formers into high molecular weight compounds. The present invention has been accomplished on the basis of this finding.